This invention relates to a method for generating glow-polymerisates of monomeric hydrocarbons and/or fluorocarbons in layers on a substrate by means of a high-frequency, low-pressure glow discharge.
Glow polymerization in which, starting from gaseous organic monomers, polymers are generated on a substrate by low-pressure plasma excitation, permits the preparation of thin uniform layers without pores. This technique is also useful in the formation of polymers from monomers which cannot otherwise be polymerized. A particularly advantageous technique for generating low-pressure plasmas for glow polymerization is high-frequency discharge, especially discharge in the radio-frequency range (see A. T. Bell in S. Veprek, M. Venugopalan, "Plasma Chemistry III", Springer-Verlag, Berlin Heidelberg, 1980, pages 43 ff) i.e., in the range between 0.1 and 100 MHz (RF), and in the microwave range (see: "J. Macromol. Sci.-Chem." A 14(3), pages 321 to 337, 1980), i.e., in the range between 0.1 and 1,000 GHz (MW). By this technique, energy can be coupled into the plasma (i.e., into the reactor in which the glow polymerization is carried out) without difficulty via external lines, capacitively or inductively and also via hollow conductors such as resonator cavities, so-called "slow wave structures" etc., whereby a reactive interaction between the electrically conducting parts and the plasma is prevented.
An interesting use of glow polymerization is in the preparation of layers from hydrocarbons (CH) and fluorocarbons (CF). In such a case, glow polymerisates with properties which can be specified over a wide range, can be produced from the organic monomers employed and which comprise a broad spectrum of polymerizable compounds. In this manner, polymer films can be prepared from CH and/or CF which possess highly desirable properties such as low-loss dielectrics, coatings which have low surface energies and are therefore liquidrepellent, sensitive resist layers for dry-structuring processes in X-ray and electron beam lithography, thin-film diaphragms for electromedical applications and electrets for various applications. These films can be produced with qualitatively and quantitatively high yields.
However, it is very difficult to avoid the formation of unsaturated structures and radicals during RF glow discharge which are incorporated into the CH or CF layers and then are reacted oxidatively in the presence of air or oxygen, or possibly also react with other reactable compounds. This in turn gives rise to chemically changed behavior of the generated layers due to undesired or undefined polarity. For instance, lyophility in the action of solvents, or substantial changes of the properties in the presence of moisture may result. This fact is due to the circumstance that some of the H or F atoms are split off from the monomer molecules in RF plasmas through collisions with electrons and are carried away by the gas stream of the monomeric component, and are thereby unavailable for polymerization and layer generation. Due to the resulting H/C or F/C ratios, which are smaller than 2 and which are understoichiometric in comparison to the monomers (see in this connection; E. Kay, J. Coburn and A. Dilks in S. Veprek, M. Venugopalan, "Plasma Chemistry III", Springer-Verlag, Berlin Heidelberg, 1980, page 1 ff) there remain in the layers primary free valences of the C-atoms which result in the formation of unsaturated structures and frozen-in radicals which cause chemical instability.